Vespa velutina is an invasive Asian hornet whose presence was recorded for the first time in France (Lot-et-Garonne) by Haxaire et al. (2006). The hornet lineage introduced to France belongs to the variety nigrithorax, which is predominantly brown coloured. The invasive population was generated by founder females which could have arrived with Chinese pottery imported by a horticulturist from Lot-et-Garonne who noticed since 2004 the presence of this species around his property. From there, the hornet rapidly spread across France and now half of the country is invaded as well as several other countries (Spain, Portugal, Belgium) where the arrival of this new alien was recently noted (Villemant et al., 2006a; Villemant, Barbet-Massin, et al., 2011; Rome et al., 2012, 2013).

The genus Vespa includes 22 species all found in Asia. They mainly occupy Central to South-East Asia whereas only a few reached the Philippines islands or New-Guinea. Hitherto, only two species naturally spread from Asia to Europe: the European hornet Vespa crabro Linnaeus, 1758 and the Oriental hornet Vespa orientalis Linnaeus, 1771 (Matsuura & Yamane, 1990). Whereas V. crabro is present all over Europe, V. orientalis only reached Bulgaria, Greece and Southern Italy. This last species is also the only hornet present in North-Africa (Carpenter & Kojima, 1997; Rortais et al., 2010; Villemant, Barbet-Massin, et al., 2011).

Like the European hornet, Vespa velutina builds a huge nest of chewed paper composed of several cell-combs surrounded by an envelope made of numerous beige and brown striped shell-like structures. The entrance hole is small and open on the lateral face of nest while it is large and basal in the nest of the European hornet. The primary nest of the Yellow-legged hornet is generally founded in a sheltered place (empty hive, hut, hole wall, roof edge, bramble bush...), however, as in the case of the European hornet, when the environment becomes adverse or the primary site too narrow for the growing nest, the colony relocates after building a secondary nest in a more open, higher location. Built in an open space, the nest is spherical when its size does not exceed 60cm in diameter. However, it becomes pear shaped and can reach up to 1m in height and 80cm in diameter when fixed in a tree crown more than 15m above ground, as it is generally usual from summer (Villemant et al., 2006b; Rome et al., 2011). When the nest is in tree, the presence of the colony can be detected only by the coming and going of workers. In fact, its flight is less noisy than that of the European hornet. A more discrete flight and nests often hidden in tree crowns may explain why the presence of V. velutina in new countries is generally only noticed several years after its arrival, as observed in the Lot-et-Garonne and Gironde départements at the beginning of the invasion, or later in the Côte-d’Or (Rome et al., 2009). The nests are most often discovered in winter, after leaf fall (Villemant et al., 2006b; Rome et al., 2011).

The European hornet, Vespa crabro Linnaeus, 1758, has a larger body than the Asian hornet with red, black and yellow colours. Its yellow abdomen is black banded. Its nest always has a wide basal entrance; it is generally built in a hollow trunk, a shed or another shelter or ground cavities, but never at the top of the tree.

The Median wasp, Dolichovespula media (Retzius, 1783), is the darkest among European wasps. It is smaller than the European hornet (1.5 – 2.2cm in length) and black coloured with fine yellow markings. Its black abdominal segments are finely margined with yellow apically. Legs are more largely yellow than in Vespa velutina. Median wasp generally builds its nest in a bush or small tree, 1 or 2m above ground. The nest, almost spherical with a conical basis, is about 20 cm in diameter. The small basal entrance hole opens laterally.

LThe Yellow jackets : the German wasp, Vespula germanica (Fabricius, 1793), des nids de frelon ; elle est de couleur grisâtre chez V. germanica and the Common wasp, Vespula vulgaris (Linnaeus, 1758) are more largely yellow and smaller (1 - 2cm in length) than Vespa velutina. They are not easily confused with it. Their nests may reach 1m in diameter and are generally built under the ground. They are sometimes placed in open buildings and other external dark shelters. These aerial nests are more or less spherical with a small basal entrance hole. The envelope of the yellow jackets’ nests is thinner than that of hornets’ nests; is pale grey in V. germanica and more browner in V. vulgaris.

The Mammoth wasp, Megascolia maculata flavifrons Fabricius, 1775, has a hairy black body with yellow spots on the abdomen. The female has a yellow head, spiny legs and can reach 4cm in length. The male is smaller with a black head and long antennae. Adults can be seen foraging on flowers in spring. The female digs the ground to reach and lay eggs on the cockchafer grubs which are consumed by its larvae.

The Giant woodwasp, Urocerus gigas (Linnaeus, 1758), is a sawfly whose larva feed on wood. This black and yellow banded wasp can be easily distinguished from a hornet by its cylindrical body and its long entirely yellow antennae. The female can reach 4.5mm in length and has a long ovipositor to lay its eggs in tree trunks. This species is totally harmless.

The Blue carpenter bee, Xylocopa violacea Linnaeus, 1758, measures 2-3cm, is entirely black with purple-blue reflections. The female of this solitary bee builds its nest in rotten dead wood and collects pollen to feed its larvae.

Several flies (Diptera) look like hornets, notably the Hornet robberfly, Asilus crabroniformis Linnaeus, 1758, which can reach 3cm in length as well as the hornet mimic hoverflies Volucella zonaria (Poda, 1761) and Milesia crabroniformis (Fabricius, 1775). Flies generally have more protruding eyes than bees and wasps, only one pair of wings and their antennae are very short.

Vespa velutina is a diurnal species which unlike the European hornet, stops all activity at nightfall. It is known as a predator of social Hymenoptera, notably bees and yellow jackets; however, like V. crabro, it also feeds on a great variety of insects and spiders (Villemant et al., 2006b; Perrard et al., 2009; Rome et al., 2011).

Since summer 2006, beekeepers are worried to note that their hives are more often and aggressively attacked by this species than by the European hornet. V. velutina workers, hover about 30cm in front of the beehive and follow one after the other to catch returning foraging bee workers loaded with pollen. The hornet generally attacks its prey in flight and kills with a stroke of mandibles behind the head; then hanging from a support, keeps the prey’s thorax containing the nutritious flight muscles and discards the rest. The flesh pellet is then brought to the nest to feed larvae with proteins.

In Asia, V. velutina is considered to be the dreaded enemy of apiaries (Shah & Shah, 1991; Ken et al., 2005). It can destroy up to 30% of an Asian bee, Apis cerana, colony. After decimating the guard bees, hornet workers enter the hive to collect the brood. To date, in France, predation by V. velutina is generally limited to adult bees because the conformation of the hive limits hornets entering. The entrance is reduced to a narrow slit which prohibits the penetration of insects larger than bees. All observers in France agree that V. velutina is generally not aggressive towards humans, so it is possible to safely observe a nest from a distance of 4 or 5 meters. Few people have been stung while trying to destroy a nest or touching a worker inadvertently. Its sting though painful is not more dangerous than the common wasp or bee sting. However, people allergic to Hymenoptera venom should take necessary precautions (de Haro et al., 2010; Schwartz et al., 2012).

Large number of people have lived in the proximity of active nests installed on or near their homes without suffering aggression from hornets coming and going. However, one must remain extremely cautious when confronted by very large nests installed in high trees. When approaching within 5m of a hornet’s nest, the larger the colony, the greater the risk of an attack by a swarm of workers.

Like other hornet species, Vespa velutina is a generalist predator that attacks a wide range of arthropod preys including bees, wasps, flies, butterflies, caterpillars and many other insect groups as well as spiders. It may also tear out flesh pellets from vertebrate dead bodies, as well as from fish and shrimps in outdoor markets (Villemant, Muller, et al., 2011). Flesh pellets are collected to feed the larvae whereas adults only consume sweet liquids (sap exuding from trunks, honeydew, nectar, flesh of ripe fruit...) and an energetic protein-rich liquid regurgitated by larvae (Matsuura & Yamane, 1990). Workers transport sweet liquids in their crop to feed by trophallaxis their fellows remaining in the nest, including the queen and future founders. In the fall, they also eat the flesh of apples, plums, grapes etc. When consuming large quantities of ripe fruit, they can cause damage in orchards.

In autumn, the new generation of reproductive females and males leave the nest to mate. Founder females are the only ones to survive winter while males, workers and the last generation of larvae die. In spring, each founder queen builds a new nest, lays several eggs and takes care of its first brood. About one and a half month later - depending on the temperature - the first workers reach adult stage and begin replacing the queen in nest construction, colony maintenance and brood feeding. From this stage onwards, the queen spends the rest of its life laying eggs. With increasing number of workers, the activity of the colony grows significantly and the size of the nest increases reaching its maximum size in the fall.

As in all other Hymenoptera, the female offspring develop from fertilized eggs and males from unfertilized eggs. The colony only includes the queen and workers (sterile females) until the end of summer when the new generation of sexual males and females develops. The old queen dies shortly before sexual swarming. The rest of the colony declines and dies at the beginning of winter. Few colonies remain active in December. Empty nests are never reused. In early spring one can sometimes find a nest containing a few females with delayed development, trapped in the old nest by the arrival of cold weather. These females are unable to establish a colony because they are not fertilized and often have atrophied wings.
Only preliminary data on the size of the Yellow-legged hornet colonies in France have been published (Villemant, Muller, et al., 2011). Their nests appear to be 3 times more populous than those of the European hornet. The largest nests produce around 15.000 individuals from April to November and in autumn may contain around 2000 workers which raise about 500 future queens plus as many males.

Birds such as Red-backed Shrike (Lanius collurio), the Honey Buzzard (Pernis apivorus) or the European Bee-eater (Merops apiaster) are active predators of the European hornets in Europe; they are also likely to attack the Yellow-legged hornet. At the end of fall, Woodpeckers (Picus spp.) and Magpies (Pica pica)have been seen pecking the nest envelop to consume the last larvae and adults of the dying colony.

The Asian bee, Apis cerana, has developed a very effective defence strategy against hornets like V. velutina which regularly attack its colonies. The aggressor is quickly surrounded by a compact mass of workers which increase the temperature inside the ball by their wings until their enemy dies of hyperthermia! After 5 minutes, the temperature reaches 45°C and the hornet dies whereas bees can stand more than 50°C. This strategy is very effective, but when repeated too often, it leads to a weakening of the hive as bee workers spend less time supplying the colony (Tan et al., 2005).

Irrational control of an invasive species can actually facilitate its installation. This was too often the case in the past with other alien species. Invasive species like the Asian Yellow-legged hornet generally have a very strong capacity to adapt and disperse in their new environment. Control methods that have an impact on the rest of the environment may therefore weaken the local species in favour of this alien. Pending the development of new and specific control methods, the following recommendations should be taken into consideration :

- Avoid trapping female founders in spring. Vespa velutina cannot be successfully controlled during the spring season. This species produces a large number of female founders (up to 500 in average for a mature nest) but spring is the period during which their mortality rate is the highest, largely due to competition between individuals of a same hornet species. In fact, destroying several founders in spring only leaves place for others (Beggs et al., 2011). In addition, there is currently no really selective trap that only catches Asian hornets. Even the trap called "selective" by its designers has an impact on non-target insects. In fact, if insects other than hornets (too big to enter the trap or small enough to escape through the tiny holes open on the trap-side) are able to escape, their stay in the trap for even a short period can have an impact (due to excess heat, humidity, etc.) on their survival or fecundity. To have a truly effective trap, the bait used should be as attractive as possible to the Asian hornet, long lasting and non attractive to other insects. Ongoing research on this subject is being conducted by INRA in Bordeaux and IRBI in Tours.

- In the case of hives being attacked by Asian hornets and only in this case, traps with physical selection (to reduce their impact on other species) must be set up, using bait made of old wax’s fermented juice (bait which gave good results in these conditions). However traps must only be set inside the apiary . This kind of trap reduces the hornets’ predation pressure and weakens their colonies. They should generally be placed during the months from July to November.

- The destruction of colonies is the most effective method to reduce the populations of the Asian hornet. This must be done as soon as possible after discovering its nests in the environment and should be pursued until the end of November. Contrary to the European hornet, Vespa velutina is a diurnal wasp; its nests can therefore be more safely destroyed at dusk or dawn. Thus almost all the individuals of the colony may be eliminated. Destruction of nests during the day (including the use of a water hose or gun) significantly increases the risk of accidents. All the individuals flying out of the nest may not be killed and can quickly rebuild a new nest nearby; they will also remain in an agitated state during several days. If the queen survives, the colony will still produce sexual males and females; but if the queen dies, the colony will produce only males; in both cases, however, the predatory activity will continue. To date, the best method to destroy a nest is to use a telescopic stick and inject an insecticide into the nest. The nest must then be brought down and burnt to prevent birds eating insects killed by the insecticide. If the nest is accessible, it can be destroyed it without using insecticide, by blocking its entrance hole with cotton and putting the nest into a bag before removing it and killing the colony by freezing. The operator should always be equipped with a protective suit against hornets.
For maximum impact the control methods mentioned above should be used until new more efficient techniques are developed. This does not mean "remain inactive," but rather "do the best you can based on our current state of knowledge."

In its native range, the Asian hornet lives mainly in forests; however, with the extension of urbanization at forest expense, it also settles suburban areas. In subtropical climates, the variety nigrithorax is generally restricted to mountainous areas (Van der Vecht, 1957). In France, the Yellow-legged hornet preferably nests in the crown of high trees, in urban or agricultural as well as in wooded areas. It seems to take advantage of the river valleys and other open spaces to disperse, but avoids pure stands of conifers. Indeed, whereas it is present in the department of Landes, forest rangers almost never observed nests in the trees of this forest.

The area of origin of the Yellow-legged hornet extends from northern India to the southern half of China, including Taiwan, as well as in Indochina (Thailand, Laos, Vietnam), Malaysia and the Indonesian archipelago. The variety V. velutina nigrithorax lives in northern India (Darjeeling, Sikkim), Bhutan and China (Carpenter & Kojima, 1997). It was reported for the first time in Korea in 2006 (Kim et al., 2006)(Kim et al., 2006). In continental Asia, it lives in climates similar to those of Europe.

• Claire Villemant, Quentin Rome, Franck Muller, Adrien Perrard et Jean Haxaire, Muséum National d’Histoire Naturelle, Paris.

Sheet prepared by

C. Villemant et Q. Rome UMR7205 CNRS-MNHN J. Haxaire (attaché MNHN) Muséum national d'Histoire naturelle Département Systématique et Evolution Entomologie, Case Postale 50 45, rue Buffon 75005 PARIS

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